Theoretical analysis of evaporative cooling of classic heat stroke patients.

Heat stroke is a serious health concern globally, which is associated with high mortality. Newer treatments must be designed to improve outcomes. The aim of this study is to evaluate the effect of variations in ambient temperature and wind speed on the rate of cooling in a simulated heat stroke subject using the dynamic model of Wissler. We assume that a 60-year-old 70-kg female suffers classic heat stroke after walking fully exposed to the sun for 4 h while the ambient temperature is 40 °C, relative humidity is 20%, and wind speed is 2.5 m/s-1. Her esophageal and skin temperatures are 41.9 and 40.7 °C at the time of collapse. Cooling is accomplished by misting with lukewarm water while exposed to forced airflow at a temperature of 20 to 40 °C and a velocity of 0.5 or 1 m/s-1. Skin blood flow is assumed to be either normal, one-half of normal, or twice normal. At wind speed of 0.5 m/s-1 and normal skin blood flow, the air temperature decreased from 40 to 20 °C, increased cooling, and reduced time required to reach to a desired temperature of 38 °C. This relationship was also maintained in reduced blood flow states. Increasing wind speed to 1 m/s-1 increased cooling and reduced the time to reach optimal temperature both in normal and reduced skin blood flow states. In conclusion, evaporative cooling methods provide an effective method for cooling classic heat stroke patients. The maximum heat dissipation from the simulated model of Wissler was recorded when the entire body was misted with lukewarm water and applied forced air at 1 m/s at temperature of 20 °C.

Model of human/liquid cooling garment interaction for space suit automatic thermal control.

The Wissler human thermoregulation model was augmented to incorporate simulation of a space suit thermal control system that includes interaction with a liquid cooled garment (LCG) and ventilation gas flow through the suit. The model was utilized in the design process of an automatic controller intended to maintain thermal neutrality of an exercising subject wearing a liquid cooling garment. An experimental apparatus was designed and built to test the efficacy of specific physiological state measurements to provide feedback data for input to the automatic control algorithm. Control of the coolant inlet temperature to the LCG was based on evaluation of transient physiological parameters that describe the thermal state of the subject, including metabolic rate, skin temperatures, and core temperature. Experimental evaluation of the control algorithm function was accomplished in an environmental chamber under conditions that simulated the thermal environment of a space suit and transient metabolic work loads typical of astronaut extravehicular activity (EVA). The model was also applied to analyze experiments to evaluate performance of the automatic control system in maintaining thermal comfort during extensive transient metabolic profiles for a range of environmental temperatures. Finally, the model was used to predict the efficacy of the LCG thermal controller for providing thermal comfort for a variety of regiments that may be encountered in future space missions. Simulations with the Wissler model accurately predicted the thermal interaction between the subject and LCG for a wide range of metabolic profiles and environmental conditions and matched the function of the automatic temperature controller for inlet cooling water to the LCG.

Automatic control of thermal neutrality for space suit applications using a liquid cooling garment.

BACKGROUND: Thermal control in the EVA spacesuit requires attention from the astronaut which is not always desirable or feasible. Improvements in thermal control involve implementation of an automatic thermal control system operating independently of the knowledge of the working astronaut. METHODS: A control system was designed, developed, and tested to automatically maintain a subject's thermal neutrality while wearing a liquid cooling garment (LCG). Measurement of CO2 production as an indication of metabolic rate was used as a signal to initiate the control response. Mean body temperature, computed as a function of ear canal temperature and mean skin temperature, provided feedback to account for the thermal state of subjects as they were being cooled by the LCG. The control algorithm was tested on nine subjects, six males and three females, who performed a varying 90-min metabolic profile using an arm cranking ergometer. A total of 27 tests, three for each subject, were conducted in a thermal chamber at three different environmental temperatures: 10 degrees C, 18.3 degrees C and 26.7 degrees C. RESULTS AND CONCLUSIONS: Evaluation of subjective comfort rating and quantitative energy storage indicates good performance of the controller in maintaining thermal neutrality for the subject over a wide range of environmental and transient metabolic states. Measurements of metabolic rate effectively initiated controller response, and feedback of mean body temperature to the controller proved very capable of accounting for various steady-state environmental conditions and inter-subject variability.

Pennes' 1948 paper revisited.

A paper published by Harry H. Pennes in Volume 1 of the Journal of Applied Physiology defined the theoretical basis for a considerable body of analysis performed by many investigators during the ensuing half century. However, during the past decade, the Pennes' model of heat transfer in perfused tissue has been criticized for various reasons, one of which is that his own experimental data seemed to be at variance with the model. More specifically, the shape of the mean temperature-depth relationship measured by Pennes was distinctly different from the shape of the theoretical curve. In this paper, I show that Pennes used an inappropriate procedure to analyze his data and that, when the data are analyzed in a more rigorous manner, they support his theory. Additional support for Pennes' theory is provided by the experimental data of H. Barcroft and O. G. Edholm [J. Physiol. (Lond.) 102: 5-20, 1942 and 104: 366-376, 1946], who had previously studied cooling of the forearm during immersion in water at various temperatures.

An analytical solution countercurrent heat transfer between parallel vessels with a linear axial temperature gradient.

Presented in this paper is a solution for countercurrent heat exchange between two parallel vessels embedded in an infinite medium with a linear temperature gradient along the axes of the vessels. The velocity profile within the vessel is assumed to be parabolic. This solution describes the temperature field within the vessels, as well as in the tissue, and establishes that the intravessel temperature is not uniform, as is generally assumed to be the case. An explicit expression for the intervessel thermal resistance based on the difference between cup-mixed mean temperatures is derived.

Immersion cooling: effect of clothing and skinfold thickness.

Accidental immersion often involves the threat of death due to hypothermia. Clothing to control heat loss in water is generally selected to minimize bulk while providing the necessary protection. While water temperature (Tw) and possible immersion time are often considered, another relevant variable is the insulation provided by subcutaneous fat. This paper describes the use of a sophisticated computer model to explore the interactions among skinfold thickness (6-20 mm mean weighted value), clothing insulation (0.06-0.23 clo, immersed), and Tw (0-20 degrees C), in producing critical hypothermia (arterial temperature less than or equal to 34 degrees C). Results indicate that subcutaneous fat strongly affects heat loss even with heavy clothing. Discussion includes examples of the possible use of skinfold data to improve specification of protective clothing for groups and allow special clothing prescription for individuals.

Influence of hypothermia and circulatory arrest on cerebral temperature distributions.

A finite element model of the bioheat transfer equation has been developed to simulate the temperature distribution in the head of a subhuman primate. Simulations were made of the induction of deep hypothermia and of subsequent hypothermic circulatory arrest (HCA). Simulations of the circulatory arrest phase were performed with different values of surface heat transfer coefficient and tissue metabolic heat generation. Numerical results were compared with experimental data for the same procedure. The simulations indicate the brain cools rapidly to a near isothermal condition in response to an infusion of cold arterial blood. However, extracerebral structures cool much more slowly. The bulk of heat gain by the brain during HCA is due to heat transfer from these warmer extra-cerebral tissues. These results suggest extended cooling by cardiopulmonary bypass (CPB) combined with surface cooling pads should reduce or even prevent the rise of brain temperatures during HCA.

Errors involved in using thermal flux transducers under various conditions.

Commercially available sensors are being used by several investigators to measure thermal flux through the skin and skin temperature at a given site. Since these transducers place an additional thermal resistance into the system, they perturb the quantities that are being measured. This problem has been analyzed theoretically to obtain the following relatively simple equations: (Q0 - Q)/Q0 = EQ = QRt/(Ta - Te) and (Ts - Ts,0)/(Ta - Te) = [EQ/(1 - EQ)] [1 - EQ2 - (Ts - Te)/(Ta - Te)] in which Q = measured thermal flux; Rt = thermal resistance of the transducer; Ta, Ts, and Te = deep tissue, skin, and environmental temperatures, respectively; and the subscript 0 denotes unperturbed values. These equations can be rearranged easily to obtain improved estimates for the unperturbed values, Q0 and Ts,0, using the measured values, Q and Ts. Use of these relationships to estimate errors for various conditions previously reported in the literature reveals that the EQ can be as large as 10% to 20% for nude subjects in hyperbaric heliox or water, and the error in skin temperature can exceed 1 degree C. When used under a 1-clo garment, the transducer will perturb Q by 4% and Ts by 0.3 degrees C.